The term “heap” as used herein is understood to describe material that has been crushed and agglomerated and stacked mechanically in a pile.
The term “air” as used herein is understood to mean atmospheric air with or without modification of the gas composition.
The term “dump” as used herein is understood to describe material, such as run-of-mine material, that has been directly discharged from a truck into a pile.
The present invention is described in the context of bioleaching copper-bearing sulfide minerals to recover copper. However, the present invention is not limited to bioleaching this material and to recovering this metal and extends to bioleaching any material in a heap/dump that requires air to be delivered to the heap/dump.
Historically, copper was produced in the 16th century at both the Harz Mountains in Germany and Rio Tinto in Spain using bacteria-assisted leaching. The role played by the bacteria was not known to the metallurgists of the time. During the 1960's Kennecott Copper Company led a research and operational program to understand the role of aeration, solution chemistry, dump design etc. and this understanding expanded the application of heap and dump leaching. Also, the need to stop acid mine drainage and the development of biooxidation technology for refractory gold contained in sulfides has expanded the understanding of the role of microorganisms and this knowledge is available to the copper industry. Even with all these advancements bioleaching technology is still in its infancy and, to optimize it, there is a need to understand fully the interaction of the biological, chemical, fluid, mass and heat transfer phenomena.
Bioleaching is growing in importance for the production of copper because of the need for environmentally friendly technology that is simple to implement and offers both considerable capital and/or operating cost savings. However, the application of bioleaching has not been easy and a lack of understanding of the key issues has caused industrial projects to fail to meet the designed production and/or delays in reaching the design capacity.
Ferric ions are an effective oxidizing agent at ambient conditions for the oxidation of copper-bearing sulfides in order to release copper into a soluble and thereafter recoverable form.
Oxidation of ferrous ions to ferric ions involves the following reaction:

It can be seen from the above equation that oxidation of ferrous ions to ferric ions is not possible in the absence of acid and oxygen.
Bacteria such as Thiobacillus ferrooxidans, Leptospirillum ferrooxidans, and Thiobacillus thiooxidans catalyse oxidation of ferrous ions to ferric ions at a rate 106 times faster than via gaseous oxygen alone.
The bacteria are unicellular microorganisms requiring oxygen, carbon dioxide for the synthesis of organic compounds, traces of nutrients (ammonium, magnesium, calcium, potassium, sulphate, and phosphate ions) for their metabolic functions, an acidic environment, and a suitable temperature. The lack or absence of the above parameters decreases the bacterial activity and causes a decrease in the oxidation rate of copper sulfides resulting in less copper dissolution.
Biological leaching of sulfides requires air.
Initially heap/dump plants relied on natural advection but this was found to be inadequate.
In recent years plants have moved to air injection. As the heaps/dumps are usually very big this has to be done cheaply and the general solution has been to blow low pressure (typically 1-3 psi) air through corrugated HDPE pipes which are buried in the ore or in inert overliner material under the ore. In a typical industrial application the pipes are usually long, upwards of 500 m in big plants, and have air holes every 1-4 m along the length of the pipes in order to distribute air in the heaps/dumps. The air holes are usually small (1-4 mm) and the experience of the applicant is that the holes tend to become blocked very quickly. Blocking of air holes is caused by fine solids and precipitates/crystals which are carried to the air pipes by the leach solutions percolating through the heap/dump.